Lot's of knowledge shared already but here is my 0.02
The main attraction for FS speakers is the idea of running "full range"
Usually this means covering the low end of the freq spectrum as already has been mentioned.
However, it has to be said that not all FS designs are full range.
In fact rarely do most FS designs go below 35Hz - 40Hz
Below 30Hz to 15Hz is true sub woofer territory and not many subs do it well.
Even then integrating a sub with the main speakers is a challenge.
It is for this reason mainly that you find many just opting out of using a sub.
So to answer the OP - if by "seniors" you mean seasoned audio enthusiasts;
then basically the complexity of sub integration is one reason why they do not use subs.
For the same reason, many also do not use FS due to bass bloat from the room conditions.
In the end the biggest factor in choosing a speaker design is the room.
If you have a large room which needs to be loaded properly then a FS design will work
If you have a smaller room with room modes then a bookshelf/standmount generally works better.
Technically adding sub(s) works for both FS and BS designs but care needs to be taken with sub integration.
Usually adding more than one sub is better than using just one sub.
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